1. Field of the Invention
This invention relates to a method of forming an electrically conducting layer over an electrically conducting surface with an electrically insulating layer being interposed therebetween. The present invention is also related to a method of forming a thermosetting resin layer on a surface of a substrate.
2. The Prior Art
When an electrically insulating layer is provided over an electrically conducting surface, such as in manufacturing multi-layered, printed circuit boards or in effecting electromagnetic shielding of printed circuit boards, there has been hitherto employed a method in which a coating composition containing a phenol resin, an epoxy resin or the like resin is applied on a surface to be insulated and the resulting coat is dried and thermally treated. The conventional method has a problem because it is difficult to form an insulating layer having a large thickness and having no pin holes or like coating failure. Thus, it is necessary to repeat the coating and curing treatment a number of times. This is time consuming.
Japanese Published Unexamined Patent Application (Tokkyo Kokai) No. 63-154,780 discloses a method wherein an adhesive composition containing (I) a mixture of (a) a thermosetting epoxymethacrylate resin obtained by addition polymerization of methacrylic acid with an epoxy resin (b) a copolymerizable cross-linking agent and (c) a thermal polymerization initiator, (II) a mixture of an epoxy resin and a curing agent, and (III) a filler and a thixotropic agent is applied to a substrate, the applied coating being subsequently heated at a low temperature to cure the ingredients (I) and then at a high temperature to cure the ingredients (II). This prior art publication neither recognizes the problem of formation of pin holes nor suggests the solution thereof.